We have been done a disservice on multiple fronts, not the least of which is by the vast majority of coaches out there who espouse repeated bouts of interval training per week or prescribe a large volume of interval training on their athletes’ hard days. The premise is that we get faster by training harder. This is true – to a finite point. In order to better understand how we can extract more from our training and how to best balance aerobic with anaerobic efforts, it’s first important to understand how the body works.
At a simplified high-level, there are 3 energy systems from which the body draws during any exercise of any duration and any intensity – phosphate, lactate and aerobic. Phosphate is mainly leveraged in all out efforts lasting up to 30sec but typically falling in the 3-10sec range. Lactate gets produced in greater quantities the closer we get to our LT (Lactate Threshold) and when we exercise above it. Lactate is a fuel source, a byproduct of our effort that our body processes more quickly than it is produced until such time as we hit and cross our LT. The aerobic energy system is when carbohydrate burning is slowed (but not stopped) and we recruit some stored fat as fuel because we are training or racing at an effort that is well below our LT.
Even 5k runners use predominantly the aerobic energy system -- as high as 93%. We really don’t elicit a high anaerobic utilization until we dip below 2min in effort, and leveraging a higher percentage of lactate for fuel instead of aerobic energy does not even occur until the efforts duration falls to less than a minute. O2 is the fuel of muscles, and it takes about 30sec for the O2 we breathe in to be put to use. People think the O2 is used immediately because when they hold their breath for a long time and the lungs start burning, they take another breath and feel instant relief. But that relief actually comes from expelling CO2, not inhaling O2. A build-up of CO2 -- because we can't inhale enough O2 to replace it -- is what becomes the limiter with higher intensity performance. When CO2 production outpaces our ability to intake fresh O2, there’s a cascading effect with higher lactate production and other internal triggers that tip us further into an anaerobic state.
So, on the one hand, it is imperative we become aerobically efficient. We do this by tuning our aerobic system so we have a turbo engine rather than a lawnmower engine. By focusing on aerobic training, we can push out metabolic triggers that start tipping us toward anaerobic system/fuel utilization (lactate, phosphate). When these triggers do ultimately get pulled, carbohydrate utilization speeds up and we burn through muscle glycogen faster. Hence, the more aerobically efficient we are, the longer our muscle glycogen lasts. Also, the less our muscles fatigue, the longer our muscle neurons fire properly and spare us from potential cramping (IMHO, cramping isn't primarily due to dehydration or salt loss; it stems from fatigue that keeps the muscles from firing properly).
But, here's the rub.
Part of our ability to become more aerobically efficient (more O2 to the working muscles) gets stimulated by bouts of high intensity training, especially VO2max efforts (lasting up to a handful of minutes). Higher intensity training forces the body to create more plasma and red blood cells, which in turn saturates the blood with a higher amount of O2. Think of a VO2max training block when the first workout of 3min intervals leaves you heaving and out of sorts. After a few weeks, the 4-5min intervals are hard and very taxing, yet there is also a sense that the body is settling in to them. This is due to the adaptation I just mentioned.
The key is to stimulate thru high intensity enough, but not too much. Too much high intensity disrupts your ability to make that internal metabolic shift to being more aerobically efficient. Doping aside, bike racers can dig deep and throw down attack after attack at the end of a 5+ hour race not because they do a lot of anaerobic training but rather because they are aerobically efficient enough that when it's "go time", they have the muscle glycogen reserves to support those efforts and their muscle neurons are not too fatigued so cramping typically (but not always) isn't an issue either. Next time you watch a bike race or mountain stage of the Tour, instead of focusing on the strongest rider, instead focus on those in the front group who inevitably start to fall away. Watch the change in their body language as steam and smoke start to inevitably pour out from under their hoods like a car overheating on the side of a desert road.
The aerobic focus -- and strict adherence to the parameters of those workouts (L2 watts, HR maxed at 70-78%) is what allows you to compete in a multi-hour race at a very consistent effort and pace. That's aerobic efficiency. It is the complement of the high intensity work -- a little but not too much -- which allows your finish time to be considerably faster than it would otherwise be. The body is a potpourri of response to stimuli. This training stuff would be so much simpler if it weren't, if instead it was more the case of pulling one trigger or another to elicit the precise response we want. But, then, I'd be out of a coaching job. :-)
So, when you think about your own training or the training program your coach has you doing, what percentage of time is spent doing intervals at LT or greater intensity? If it is consistently more than 10% per week, it is arguably too much. When we look at our lactate production curve, the biggest limiter to performance is how fast we can go before the curve starts to steepen up and to the right – meaning, when lactate production starts trending toward a greater amount being produced than can be processed causing the body to pretty quickly shut down. The longer the “tail” of our lactate curve, the more aerobically efficient we become and, thus, the faster we also become.